Double-circle shield tunneling technology is a brand-new construction technology, and the rotation angle produced by shield machine in the construction process has an extremely important influence on the construction. Through construction practice, the causes of corner are analyzed, and corresponding construction countermeasures are formulated to reduce the various influences brought by corner. The corner rectification measures were applied in the construction of M8 line, which achieved good results and enabled the construction to proceed smoothly.

Key words: Corrective measures for the causes of corner bend in double-circle shield construction technology 1. General situation of the project With the continuous development of urban construction, the number of subway construction lines is also increasing. This puts forward higher requirements for the current subway construction equipment and its supporting construction technology, and it is imperative for the double-circle shield to be put into use. Double-circle shield is very similar to ordinary single-circle shield in appearance, and two tunnels can be completed with one push. 198 1 year, Japan came up with the idea of using DOT for tunnel construction. 1987 carried out the field test of horizontal two-in-one polka dot shield, and successfully registered the patent of polka dot construction method. 1988, field test of longitudinal two-in-one double circular shield was conducted in Japan; 1989, the world's first double-circle shield project was constructed in Hiroshima, Japan. The tunnel between Huangxing Greenland Station and Xiangyin Road Station on M8 line of Shanghai rail transit is the first tunnel in China to be excavated by double-circle shield machine. The joint venture between Shanghai Tunnel Engineering Co., Ltd. and Japan Dafeng Construction Co., Ltd. is responsible for the construction. Excavation of this section of tunnel officially started on August 8, 2003, and was completed on June 65438+February 3 1 day, 2003. Therefore, China has become the second country in the world to use double circular shield machine to complete tunnel excavation. The tunnel in this section is 868m long, and the external dimensions of the tunnel are? Φ Φ 6300mm×10900mm (outer diameter× width), and the center distance of double lines is 4600mm；; The maximum slope of the tunnel is 28‰, the radius of the least square curve is 495m, and the thickness of the tunnel covering soil is 5.2 ~ 12m. The tunnel lining adopts precast reinforced concrete segments, which are assembled by staggered joints. Each ring liner consists of 8 circular segments (a), 1 big gull segment (b), 1 small gull segment (c), 1 cylindrical segment (d) * * *1segment, with a segment thickness of 300. The segments are connected by bolts in the longitudinal direction and the circumferential direction, and waterproof joints are made of water-swellable rubber waterstops.

Second, the control of the rotation angle of double-circle shield In the construction of double-circle shield, the technical measures such as the control of shield axis (plane and elevation) are not much different from those of single-circle shield, but the big difference between double-circle shield and single-circle shield is the deflection of shield. When the single circular shield is advancing, even if it deflects, it will not directly affect the completed tunnel; However, in the process of double-circle shield propulsion, the height difference between the left and right sections and the inclination of the column will directly affect the axis of the tunnel. Axis control (plane and elevation) of shield is a mature construction technology. However, in the corner correction measures, the double circular shield adopts a set of unconventional correction measures. 1, the corner can be divided into two parts: the internal cause is caused by the characteristics of the shield machine itself, and the external cause is caused by the external soil conditions and construction. (1) Left and right differences of shield machine fuselage There are some differences in the left and right structure and equipment configuration of double-circle shield machine fuselage, such as the setting position of pedestrian door and the structure of left and right assembler, which leads to uneven weight of shield machine. (2) Manufacturing accuracy of shield machine In the manufacturing process of double-circle shield machine, if the left and right shells are deformed, it is easy to produce deflection. (3) In the process of double-circular shield propulsion with uneven surrounding soil, due to the inhomogeneity of soil, when the cutter head cuts the soil, the moment of the left cutter head and the right cutter head is different, which leads to the moment difference of the shield machine, which leads to the deflection of the shield machine; In addition, the shield machine has different soil quality and different foundation bearing capacity, which will also cause shield machine deflection. (4) Curved section construction or deviation correction When the double-circle shield machine is in curved section construction or axis deviation correction, the force and direction on the left and right sides will be different, which will lead to the deviation of the shield machine; In addition, if there is local overbreak, it is easy to cause shield deflection. 2. The adverse impact of the corner of the double circular shield will have a considerable impact on the posture of the shield and segment, the forming quality of the tunnel and the ground settlement. (1) The posture of shield and segment is not good. If the rotation angle generated by double circular shield machine reaches 0.6 (36'), the left-right elevation difference will be 48mm, while the elevation deviation of ordinary subway tunnel is 50 mm ... So once a large shield angle is formed, the elevation may exceed the design requirements. After the shield turns, the segments gradually deflect in the same direction, resulting in poor segment posture, and the correction of segment deflection is more difficult than that of shield machine. When the relative angle between shield machine and segment increases, the gap between them will become smaller, and the difficulty of segment assembly will increase greatly. It is difficult to assemble columns when assembling segments of double-circle shield. If the assembly gap becomes smaller, the column can only be inserted forcibly, which will easily cause segment fragmentation to a large extent and affect the tunnel quality. (2) Land subsidence is not easy to control. In the construction of double-circle shield, the soil in the shield chamber is directly communicated with the front stratum, which makes the relationship between the set earth pressure on the front of the shield and the stratum deformation at the cut of the shield more direct. When the rotation angle of double circular shield is modified, the earth pressure on the left and right parts of the shield is different, resulting in uneven ground settlement. In addition, in order to correct the shield deflection, it is necessary to open a copying knife to dig over the soil, and the resulting building gap may form a large ground deformation if it is not filled with grouting in time. (3) Affecting the use of shield equipment Due to the corner of the double-circular shield, various equipment and devices that were originally in a relatively reasonable position moved, resulting in poor use or even failure of equipment. For example, there is a gap between the real circle retainer and the screw machine on the assembly platform of the shield machine, which does not affect each other when the shield machine advances. However, when the corner appears and is not corrected for a long time, coupled with the vibration of the machine, the screw machine moves slightly, resulting in no room for the real circle holder to move. (4) Tunnel Forming Quality The corner produced by the double circular shield will cause the height difference between the left and right tunnels and the inclination of the columns, which will directly affect the stress and quality of the completed tunnels. If the elevation of the double-circle shield machine itself is not well controlled, the corner will cause elevation deviation, which may affect the building clearance of the tunnel. When the shield has corners, the correction jacks are usually open, and the jacks are grouped to facilitate shield correction. However, if the jack is not used properly, the block is easy to break. 3. Correction measures of shield rotation angle Because the left and right cutter heads of double-circle shield rotate reversely, it is impossible to correct the rotation angle by rotating the cutter heads back and forth like a single-circle shield machine, but a set of unconventional correction measures are adopted. The maximum allowable design angle of M8 line double circular shield machine is 0.6 (36'), and the control range of general shield angle in actual construction is 0.3 (18'). (1) correction jack The correction jack itself is a shield propulsion jack, which is mainly arranged on both sides of the shield machine. When rectifying the deviation, the cylinders of jacks can be adjusted to a certain extent along the circumferential direction, so that the deviation can be rectified by using the component of jack thrust in the circumferential direction (IHI double circular shield machine used in M8 line is equipped with 12 rectifying jacks, 6 on the left and 6 on the right, symmetrically arranged, and the maximum deflection angle of jacks is 1). When using the rectification jack, the propulsion jack should be opened as far as possible (to ensure that the segment is less affected by the circumferential component of the rectification jack), and the rectification jack should be adjusted several times (which has obvious effect on correcting the shield angle). However, when the deflection correction jack is used for correction, the reaction force will inevitably cause the segment to rotate in the opposite direction. Therefore, if necessary, synchronous grouting should be carried out diagonally at the upper and lower parts of the left and right line tunnel segments according to the rectification direction, which can not only ensure the rectification effect of the shield, but also keep the segment posture in good condition. In addition, when using the rectification jack, always pay attention to the deflection position of the jack to prevent the displacement of the jack from affecting the rectification effect. In a word, we should be careful when using the rectification jack, so as not to adversely affect the tunnel forming quality. (2) Jack grouping Because the width of the double-circular shield is large, different jack grouping forms can be adopted to correct the deviation to a certain extent on the premise of satisfying the shield posture (plane and elevation). For example, when the shield machine turns right (clockwise), open the upper jack on the left and the lower jack on the right as much as possible to achieve the purpose of rectifying the deviation, but avoid causing segment fragmentation. (3) Overexcavation with copying knife can change the deflection direction of shield machine by using copying knife to overbreak the corresponding parts. When the shield machine turns left, open the copying knife to dig the upper left part and lower right part of the shield machine (see figure 1), resulting in a certain construction gap, which is beneficial to correct the deflection of the shield machine. When using the copying knife, the synchronous grouting amount should be increased according to the overbreak amount.

Figure 1 Schematic Diagram of Overexcavation with Copying Cutter

(4) Control the amount of excavation. The purpose of controlling the rotation angle of the shield is achieved by using the different excavation amount of the left and right screw machines. On the premise of ensuring that the ground at the cut of the shield is not affected, the excavation amount of the left and right screw machines is adjusted by expanding the speed difference between the left and right screw machines. On the side of the shield machine with higher elevation, the amount of excavation should be correspondingly more. According to the construction experience of double circular shield engineering on M8 line, in order to make the shield turn left, it is necessary to excavate more on the left side, and at the same time, the lower left side and the upper right side should cooperate with the copying knife to overbreak. (5) When the single-side ballast shield deflects, it is simple and practical to pressurize the segment or lead block and other heavy objects on the side with higher elevation of the shield to make the shield rotate (if the segment posture is not good, it can also be pressurized in the single-side tunnel to realize segment rotation). (6) When the cutter head steering shield machine is running, due to the different soil, the torque of the left and right cutter heads is different, which has different deflection effects on the shield machine. Therefore, the deflection can be corrected by changing the rotation direction of the cutter head in combination with the deflection direction of the shield machine.

Third, the effectiveness of the use of corrective measures In actual construction, the corrective measures for shield corners should be used separately or in combination according to the actual situation (combined with soil conditions). For example, M8 line 1 10 ring ~ 13 1 ring is basically in ④ gray muddy clay, and the rotation angle of10 ring is-12' (the angle is negative when the shield turns left). During the propulsion process, the rectification jack is always in the rectification state. The effect of reducing the rotation angle of shield is achieved by using deviation correction jack and jack grouping.

For example, when the 402-440 ring is pushed (the rotation angle of the 402 ring is 2 1'), the shield is basically in the No.4 gray muddy clay, and the rectification jack is in the rectification state. After pushing to the 4 15 ring, the segment was found to be broken, so the shield jack was replaced and grouped, and the deviation was corrected by multiple excavations on the left side (that is, the speed of the left screw machine was higher than that on the right side), so as to achieve the purpose of deviation correction.

For another example, when the shield machine is pushing the 600-650 ring (the corner of the 600 ring is-18'), it is in the mixed soil of No.23-1No.23-2 and No.4. At this time, all the rectification jacks are turned on, and the shield jacks are grouped into upper right jacks and lower left jacks. The excavation amount on the right side is greater than that on the left side, and the pipe is suspended on the right side of the shield.

To sum up, in the case of good soil quality, a small amount of correction measures can achieve the correction effect. However, if the soil conditions are poor, we must comprehensively use various or even all rectification measures to achieve the purpose of rectification. However, when using corrective measures with certain side effects, corresponding auxiliary measures must be equipped to eliminate the adverse effects. When the shield machine is slightly deviated, some simple and easy measures with less side effects (such as changing the direction of the cutter head and hanging the segment unilaterally, etc.). ) to control, so that the rotation angle is always controlled within a certain range, otherwise once the rotation angle becomes a trend, it is difficult to correct it in a short time under the action of great inertia.

Four. Conclusion DOT construction method can replace double-line (bidirectional) tunnel constructed by large-section circular shield and two bidirectional (unidirectional) tunnels constructed by single circular shield, and has been used in subway tunnels and underground expressways. DOT method has been widely used in subway, municipal, energy and other projects because of its unique advantages, such as greatly improving the construction efficiency, effectively saving underground resources, reducing the impact on the surrounding environment and reducing the project cost. M8 line is the first double circular shield tunnel in China. Because this method has just started in China, some technologies are still immature and perfect. Although the correction measures of shield machine corner have been preliminarily mastered, and the tunnel construction quality is excellent, the double-circle shield machine excavation technology is a brand-new construction technology, which is different from the single-circle shield construction. In the future construction practice, we will constantly find problems, analyze the reasons, formulate countermeasures, solve problems and accumulate experience.